[css-syntax] Clarify that tokens match ASCII case-insensitive *unless otherwise specified*.

Author: tabatkins

css-syntax-3/Overview.bs

@@ -2969,8 +2969,8 @@ Defining Grammars for Rules and Other Values</h2>
 		<li>The <<colon-token>> (written as <code>:</code>), <<comma-token>> (written as <code>,</code>), <<semicolon-token>> (written as <code>;</code>), <a href="#tokendef-open-paren">&lt;(-token></a>, <a href="#tokendef-close-paren">&lt;)-token></a>, <a href="#tokendef-open-curly">&lt;{-token></a>, and <a href="#tokendef-close-curly">&lt;}-token></a>s.
 	</ul>
 
-	Tokens match if their value is an <a>ASCII case-insensitive</a> match
-	for the value defined in the grammar.
+	Tokens match if their value is a match for the value defined in the grammar.
+	Unless otherwise specified, all matches are <a>ASCII case-insensitive</a>.
 
 	<p class=note>
 		Although it is possible, with <a>escaping</a>,